Telomerase serves a crucial function in human cancer by elongating telomeres and supporting cancer cell immortalization. Telomerase is thought to be upregulated during human cancer progression, enabling aspiring cancer cells to overcome senescence barriers. Telomerase expression in normal tissues is highly enriched in stem cells and progenitor cells, and is efficiently silenced in more differentiated progeny. Despite clear evidence for restricted expression of telomerase, challenges in detecting telomerase at the single cell level have limited our ability to understand telomerase regulation in tissues and in cancers in depth. In addition to its role in telomere elongation, we have shown that TERT, the telomerase protein subunit, exerts potent effects on tissue stem cells through an auxiliary pathway. TERT modulates the output of the Wnt signaling pathway through an interaction with the chromatin remodeling protein Brg1, and a direct association with promoter chromatin at Wnt-regulated genes. TERT overexpression activates quiescent stem cells in vivo and regulates a Wnt-related gene expression program. We hypothesize that telomerase is almost universally associated with progenitor cells and with human cancers, not only to overcome senescence barriers, but also to support Wnt signaling and associated programs of self-renewal, proliferation and survival critical for progenitor cells and for developing cancers. o address these hypotheses, we have generated novel mouse knockin and knockout reagents that facilitate the dissection of endogenous telomerase at the single cell level and at the protein level, and that allow deletion of TERT in vivo in tissues and in cancers with spatiotemporal control. Remarkably, we find that acute deletion of TERT in vivo abrogates Wnt signaling and leads to immediate phenotypes not evident in germline telomerase knockout mice. We plan to pursue the following specific aims: (1) To define the identity and function of telomerase-positive cells in tissues (2) To understand the role of TERT in epithelial cancer through acute deletion and (3) To understand the function of TERT in controlling epigenetic tumor states. PUBLIC HEALTH RELEVANCE: Cancer is inherently linked to unlimited and dysregulated proliferation of abnormal cells forming a tumor that can spread to other sites in the body, eventually killing its host. This behavior of cancer cells is inherently different from most cells within the body, but does share important commonalities with stem cells. Tumor cells and stem cells have a very large capacity to divide due in part to expression of the enzyme telomerase, which elongates telomeres that special caps that protect chromosome ends. Tumor cells and stem cells also share the ability to self- renew, that is, to divide in such a manner as to generat a daughter cell with all the capabilities of the original cell. Tumor cells and stem cells also employ many of the same growth control pathways that underlie these similar traits. In this proposal, we will study how telomerase is regulated in tissues and in tumors. We will pursue a novel hypothesis that telomerase contributes to tumorigenesis by modulating growth control pathways important in stem cells, in addition to its role in elongating telomeres.